This invention relates to a nest level judging device for use in a protocol conversion system.
A protocol conversion system is for use in an information communication network between heterogeneous data processing systems. When used between heterogeneous data processing systems, the information communication network is for mutually connecting the heterogeneous data processing systems together either in a local area or between local areas to carry out information interchange between the heterogeneous data processing systems. It is expected that such an information communication network will wide spread among the society information. The information network is generally built on the basis of a network architecture systematized according to a protocol.
Inasmuch as such a protocol has been originally developed by computer makers, it is impossible to carry out the information interchange between the heterogeneous data processing systems. In order to solve this problem, a standard protocol has been established by International Organization for Standardization (ISO). This standard protocol is called a layer protocol of Open Systems Interconnection (OSI) in the art. Such a layer protocol is disclosed in a book written by William Stallings and published by Macmillan Publishing Company, New York, under the title of "DATA AND COMPUTER COMMUNICATIONS", pages 394 to 408, Chapter 12.
The layer protocol consists of seven layers which comprise a physical layer, a data link layer, a network layer, a transport layer, a session layer, a presentation layer, and an application layer. The application layer comprises a plurality of sublayers, for example, sixty-four or more sublayers. Each layer of the layer protocol has a processing function which should be standardized between the computer makers. Inasmuch as the layer protocol of the OSI is well-defined, it is possible to carry out protocol conversion between the layer protocol and a specific protocol developed by a certain computer maker. Each of the heterogeneous data processing systems comprises a protocol conversion system for carrying out the protocol conversion, by which it is possible to carry out the information interchange between the heterogeneous data processing systems.
Attention will be directed to the application layer. As is known in the art, the application layer is divisible into first through N-th sublayers, where N represents a predetermined natural number which is equal to, for example, sixty-four. The first through the N-th sublayers are referred to first through N-th nest levels in the art, respectively. The first through the N-th nest levels are a lowest through a highest nest levels, respectively. According to the application layer, data transmission is carried out between the heterogeneous data processing systems on the basis of Message Handling Systems (MHS) recommended by the International Telegraph and Telephone Consultive Committee (CCITT) Recommendation X. 409. More specifically, data are transmitted between the heterogeneous data processing systems as a plurality of tokens.
Each of the tokens has a nest level selected from the first through the N-th nest levels as a selected level. Each of the tokens comprises a header and a data set as described in Chapter 12 of the above-mentioned book. The data set has a data length and is for nesting one of the tokens that has a higher nest level than the selected level and will be referred to as a particular token. The header comprises a data length code representative of the data length and a data identifier code. The data identifier code includes a nest bit indicative of whether or not the particular token is nested in the data set.
In the protocol conversion system, the selected level must be judged. A conventional method for judging the selected level has been performed by software in computer. According to the conventional method, the tokens are memorized in a memory. The tokens are successively read from the memory to a central processing unit (CPU) as read-out tokens. The software is used in judging the selected level from the read-out tokens. The conventional method has therefore been disadvantageous in that a large processing time is consumed to judge the selected level. As a result, it is impossible to rapidly and smoothly carry out the information interchange.
In order to remove the above-described disadvantage, an improved nest level judging device is disclosed in U.S. Pat. No. 4,916,660 issued to Kousuke Takahashi, the instant applicant, and assigned to the instant assignee. Briefly, the improved nest level judging device comprises a detecting circuit for detecting the nest bit to produce a nest bit detection signal and a selection circuit for delivering the data length codes of the respective tokens as first through N-th data codes in response to selection signals. The improved nest level judging device further comprises a processing circuit for processing the first through the N-th data codes into a processed signal which is N bits long. Responsive to the nest bit detection signal and the first through N-th data codes, a level decision circuit decides a decided nest level indicative of one of the first through the N-th nest levels that is selected as the selected level. The level decision circuit supplies the decided nest level as one of the selection signals to the selection circuit.
Although the improved nest level judging device can judge the decided nest level at a high speed, it requires a large amount of hardware.